A term in the middle — Aptitude Question
A term in the middle: Find the missing term: 3, 7, 15, ?, 63, 127 Answer and worked solution on CodeKairo.
- Topic: Number & Letter Series
- Section: Logical Reasoning
- Difficulty: Medium
- Time target: 60 seconds
Question
Find the missing term:
3, 7, 15, ?, 63, 127
Options
- 23
- 27
- 31
- 35
Show the answer and the worked solution
Correct answer: C. 31
Worked solution
- The rule is × 2 + 1: 3 → 7 → 15 → ? → 63 → 127.
- 15 × 2 + 1 = 31, and 31 × 2 + 1 = 63. ✓
Answer: 31.
The idea
With a gap in the middle, verify the rule on both sides of it. That confirms the answer rather than merely suggesting it.
Shortcut. Add 1 to each term: 4, 8, 16, ?, 64, 128 are powers of two, so the blank is 32 − 1 = 31.
Common trap. 23 — adding the last gap, 8, again (15 + 8); the gaps double (4, 8, 16), so the next is 16, and 31 × 2 + 1 = 63 confirms it.